This invention relates to a method and a system for evaporating at least one substance.
There have been prior-art systems, referred to as rotary evaporators, serving for the evaporation of substances and especially of solvents in the process of concentrating or distilling pharmaceutical or chemical products. In these prior-art rotary evaporators the liquid substances to be evaporated are introduced into the interior space of a rotating flask (evaporating vessel) typically designed to rotate within a thermal bath. As the rotating flask revolves, a thin film of liquid forms on the inside of the rotating flask from which film the substances, in particular the solvent, will then evaporate. A certain amount of the substance, i.e. of the solvent, also evaporates directly from the surface level of the liquid in the rotating flask. From the rotating flask the vapor travels through a vapor conduit toward a condenser where it is reprecipitated by condensation. It is also a common practice to generate a negative pressure or vacuum in the rotating flask for the purpose of increasing the vapor pressure of the substances to be evaporated and to preserve the product. In one transition area the vapor conduit is connected to the rotating flask via a rotatable junction. That junction is sealed by means of a rotary gasket.
Disclosed in the German patent (DE) 1.224.062 is a rotary evaporator incorporating an operating flask that can be rotated around its axis of symmetry by a motorized drive unit. The operating flask can also pivot freely around a swivel pin that extends in a direction perpendicular to that axis of symmetry as it floats freely on the surface of a water bath. In a rotary evaporator incorporating that type of pendulum suspension the weight of the operating flask is thus counterbalanced only by the buoyancy of the operating flask in the water bath, which substantially reduces the torque and the forces bearing on the mount of the operating flask. The glass adapter which, together with the operating flask attached to it, can be rotated via the rotary drive, is sealed by an O-ring gasket against another, non-rotating glass adapter that is connected to the enclosure. Inserted in the stationary glass adapter on the far side from the rotary glass adapter is an inlet and scrub nozzle equipped with a two-way cock and a hose fitting for the intake of fresh liquid, as well as an offtake by way of which a scavenging fluid can be sprayed against the inner walls of the glass adapters to flush out condensation in these glass adapters.
DE 35 11 981 C2 describes a method for rotational vacuum evaporation as well as a system for implementing that method. That earlier rotary evaporator incorporates a pendulum suspension whereby a freely buoyant rotating flask floats on or in a liquid bath. The level at which the rotating flask floats relative to the surface level of the liquid in the liquid bath is measured directly or indirectly by means of a measuring device, providing an autoleveling variable within a regulating range or an actuating variable within a control range for the purpose of regulating or controlling the amount contained in the rotating flask. The height position of the rotating flask relative to the top surface level of the liquid provides a direct measure of the fill contained in the rotating flask, i.e. of the amount of liquid in the rotating flask.
That height position and thus the content of the rotating flask is determined by the amount of piped-in liquid to be evaporated which thus serves as the control parameter. A solenoid valve serves as the regulating device for opening and closing the feed of the substance to be evaporated and its opening and closing action is correspondingly controlled by a regulator, thus ensuring at all times an adequate quantity of the substance to be processed in the rotating flask so as to keep most of the inner surface of the latter coated with the substance as the flask rotates. The rotary evaporator thus operates at a high level of efficiency.
When the desired degree of thickening or concentration of the unevaporated product remaining in the rotating flask is reached, the evaporation process is shut off. If due to the evaporation the relative density i.e. volume weight of the concentrate remaining in the rotating flask has increased as for instance when concentrating a saline solution, the default setting for the height position of the rotating flask is automatically and incrementally adjusted as the volume weight of the content of the rotating flask increases. In this fashion the content, i.e. the volume of the substance in the rotating flask, is held at a constant level in spite of its progressively augmented relative density. The incremental adjustment of the setpoint value for the height position of the rotating flask can always begin with a new evaporation process for a newly filled rotating flask either as part of a series of repetitions or, alternatively, during a continuous evaporation process involving multiple metering steps for replenishing the content of the flask. Thus, in order to keep the volume of the density-gaining product constant, a somewhat larger amount of the substance is added in each progressive refilling or metered replenishing step than in the preceding refilling or metered replenishing step.
In other words, in the case of that prior-art rotary evaporator described in DE 35 11 981 C2 the content or volume of the product in the rotating flask is regulated by time-spaced metering steps or intake phases, with the dosage increasing with every step. During a metered dispensing step the solenoid valve is completely open, permitting an essentially constant product volume flow into the rotating flask. The amount of the substance added is controlled during the time in which the solenoid valve stays open. The metered dispensing step is terminated by closing the solenoid valve.
According to DE 35 22 607 A1 which is an application supplementary to DE 35 11 981 C2, the volume content of the rotating flask, or any change of that volume content reflected by the degree of immersion of the rotating flask in the bath liquid, is determined by weighing and the result of the weighing serves as the regulating or control parameter for the evaporation process. The rotating flask is kept floating by its buoyancy on or in a liquid bath in particular by virtue of a pendulum suspension or a linear raising and lowering system. As the means for controlling the evaporation process, DE 35 22 607 A1 merely mentions a regulated product feed, referring to the older DE 35 11 981 C2. The product feed takes place via a product intake valve which is opened when the rotating flask is to be filled, and closed before the evaporation process begins.
DE 43 19 498 A1 describes a rotary evaporator with a revolving vessel that is pivot-mounted along an oblique axis of rotation, and with an intake for feeding the substance to be evaporated into the revolving vessel, an offtake for removing vapor from the vessel, as well as a microwave heater for heating the evaporation substance in the revolving vessel. The feed line for the substance to be evaporated is equipped with a pump whose throughput capacity is adjustable. The feed is said to be continuous.
DE 35 96 644 A1 describes another rotary evaporator in which the metered addition of the sample solution into the rotating flask is automatically controlled via a sample metering vessel. The sample solution is moved from a storage container into the metering vessel for interim storage and subsequent release into the rotating flask. This allows for the sample solution, or the substance to be evaporated, to be fed to the rotating flask in specific, fixed, predetermined quantities. Interpositioned between the storage container and the sample metering vessel and again between the sample metering vessel and the rotating flask is a line with a processor-controlled valve serving to control the metered addition of the product into the rotating flask.
Another rotary evaporator is discussed in Patent Abstracts of Japan No. 590 90 602 A in which the amount of sample liquid contained in the rotating flask is controlled to a specific fill level or a specific volume with the aid of a sensor. When the fill level detected by the sensor drops below setpoint, an electromagnetically operated valve continuously meters out fresh sample liquid during a replenishment phase until the required fill level is restored.
In all of the prior-art rotary evaporators described above, the amount of product added during a dosing phase, i.e. the replenishment rate or product flow, is constant per unit of time. For certain products it is necessary, however, to have the ability to operate at a slower replenishment i.e. reduced flow rate per unit of time in comparison with other products so as to prevent a negative impact on the process. Examples of such problematic products include high-foaming or outgassing products, for instance some of the polymers or peptides. If one were to select the diameter of the inlet valves in these prior-art rotary evaporators small enough to suitably reduce the feed rate of these critical products, it would affect the throughput and thus the productivity of the process when other, less critical products are to be handled.